The present invention relates generally to forming polymeric materials and more particularly to a method and apparatus for forming distortion-free polymeric materials.
Polymeric materials are used in a wide variety of applications. Typically, polymeric materials are used to manufacture transparent panels such as windows or windshields for various applications including aircraft, automobiles, motorcycles, boats and the like. Such applications, especially those for aircraft, require an extremely clear, undistorted, transparent panel, which is resistive to scratching and impact in order to afford the pilot a clear view of the surroundings.
Traditionally, acrylic plastic is used to form such transparent panels. Acrylic plastic is noted for its excellent optical properties and weatherability, having outstanding resistance to the effects of sunlight and exposure to the elements over long periods of time. Subjected to long term exposure to the elements, acrylic plastic does not experience significant yellowing or any other significant changes in its physical properties. Acrylic plastic, however, does not have as high an impact strength as do other polymeric materials and thus, are less preferred for applications where impact strength is of importance.
Polycarbonate is a high-performance thermoplastic with the characteristics of high impact strength, optical clarity, heat resistance and dimensional stability. Polycarbonate, on the other hand, does not include the same weatherability characteristics of acrylic plastic. However, the transparent panels, whether produced using acrylic plastic or polycarbonate, include a hard protective coating to prevent scratching, abrasions or other markings that would reduce the service life of the transparent panel. Further, the hard protective coating protects the base sheet, whether acrylic plastic or polycarbonate, from UV degradation. As a result, the transparent panel is protected from any degradation, such as yellowing, abrasion distorting, and the like, even though the base sheet (e.g. polycarbonate) would otherwise degrade from such exposure. Therefore, it is desirable in the industry to use polycarbonate for producing transparent panels because of its high impact strength, while it remains protected from UV degradation and abrasion by the protective coating which is applied regardless of the material used.
Traditionally, polymeric sheets of acrylic plastic are formed using molds that include contoured upper and lower surfaces. The contoured surfaces define the desired shape of the polymeric sheet, directly contacting the entire upper and lower surfaces of the polymeric sheet. Because of the hardness of the upper and lower surfaces of an acrylic plastic sheet, it may be formed in this manner without distorting the upper and lower surfaces. However, the upper and lower surfaces of a polycarbonate sheet are not as hard and therefore, when heated, may be distorted upon contact during the forming process. For this reason, the use of traditional molds, which directly contact the upper and lower surfaces of the polymeric sheet, are not desirable for forming polycarbonate sheets. Traditional molds have increased potential for distorting the surfaces of the polycarbonate sheet, thus producing an increased number of rejected panels and driving up production costs.
Accordingly, the present invention provides an apparatus for forming a polymeric material, such as polycarbonate. The present invention enables forming of a polycarbonate sheet without distorting the key visibility areas of the sheet. The apparatus of the present invention provides a forming mold including a first half having a bottom wall and a first side wall defining a first interior space and a first edge and a second half having a top wall and a second side wall defining a second interior space and a second edge. The first and second halves come together to clamp the peripheral edge portions of a sheet of polymeric material therebetween for forming the sheet whereby the sheet is vacuum drawn into one of the first and second interior spaces. A cooling mechanism is disposed within one of the first and second interior spaces and a sensing mechanism is attached to one of the first and second halves for sensing a draw depth of the sheet within one of the first and second interior spaces. The first edge is preferably contoured for defining a final edge contour of the sheet and the second edge correspondingly contoured for facilitating engagement of the first and second halves. Further, the first edge is preferably beveled and the second edge correspondingly beveled for facilitating engagement of the first and second halves.
In a preferred embodiment, a trimming mechanism is provided for trimming a perimeter of the sheet to a desired shape. A retention mechanisms is also provided and operatively supported by one of the first and second halves for biasing the sheet into contact with one of the first and second edges of the first and second halves.
The present invention further provides an improved method for forming a sheet of polymeric material. The method of the present invention includes the steps of: heating the sheet to a first temperature, retaining a sheet between first and second mold halves of a forming mold, generating a vacuum on one side of the sheet thereby drawing the sheet into an interior space of one of the first and second mold halves, and cooling the sheet from the first temperature to a second temperature upon achieving a specified draw depth of the sheet within one of the first and second mold halves. The method preferably includes the step of detecting a draw depth of the sheet within one of the first and second mold halves for initiating the cooling. Alternatively, the heated sheet may be formed by use of blow air to exert a pressure on the other side of the sheet in lieu of the vacuum forming process or perhaps by use of a combination of both blow air and vacuum on opposite sides of the sheet.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.